In satellite communication or the like, a loading space and/or a loading weight of an antenna mounted on a mobile body such as a vehicle or an airplane are limited.
Therefore, the antenna is required to be small in size and light in weight.
An array antenna that transmits and receives signals using a plurality of antennas is one means for satisfying the above requirement. As an example of a conventional array antenna for the satellite communication, as in Patent Document 1 mentioned below, there is known a configuration in which a patch antenna and an antenna obtained by stacking a metal having open holes are used.
Meanwhile, an antenna is sometimes required to be usable in orthogonal double polarization.
In order to realize this requirement, as in Patent Document 2 mentioned below, there is a method of crossing two rectangular horn antennas and vertically disposing these antennas.
Further, as a simpler configuration, as in Patent Document 3 mentioned below, there has been proposed the following method: when a power feeding probe for exciting one polarized wave is disposed on a substrate, the substrates are superposed and disposed with two layers such that the respective power feeding probes are orthogonal to each other.
Though an antenna described in Patent Document 1 mentioned below is adapted to orthogonal polarization, a patch antenna is used, and even when a non-exciting element that contributes to a wider band is added thereto, in general, the band is approximately 10%, and therefore, there is a problem such that a wider band more than the above is difficult.
An antenna described in Patent Document 3 mentioned below is adapted to the orthogonal polarization, and usable in a wide band of several tens %.
However, when a plurality of the antennas are disposed as element antennas to configure an array antenna, if all the element antennas are tournament-connected, there is a problem such that a power feeding structure is complicated to increase its manufacturing costs and manufacturing processes.
FIG. 17 shows an example of a power feeding circuit of an array antenna configured by sixty-four elements in total including eight elements in an x direction×eight elements in a y direction.
Note that the figure shows a structure adapted to the polarization in the x direction. For a power feed for the polarization in the y direction orthogonal to this direction, a structure obtained by rotating the figure 90° is further separately necessary.
When the entire power feeding circuit is configured by a waveguide in order to reduce a loss in the power feeding circuit, in addition to a complicated structure, the weight and volume of the power feeding circuit increase.
As a countermeasure against this, it is conceivable to configure a part of the power feeding circuit using a strip line on the same surface as that of a power feeding probe, vertically draw a wire down to an antenna lower part, and thereafter connect the wire using the waveguide.
In the following explanation, a drawn-down section is described as a vertical power feeding section.
FIG. 18 is an example in which only portions related to the present invention are extracted from the antenna described in Patent Document 3 mentioned below and, when four elements are set as a unit, sub-arrays are configured using a strip line.
The elements of the antenna are configured from a first cavity part 201 closed in the bottom, a first excitation circuit 210 that excites a first polarized wave, a second excitation circuit 220 that excites a second polarized wave, and a third cavity part 250 having open holes.
The first cavity part 201 is composed of, for example, a metal in which openings are cut.
Note that the bottom is closed.
The first excitation circuit 210 includes a first power feeding probe 213 configured in a dielectric substrate 211 by a pair of elements to which power is fed in phases opposite to each other for each of element antennas, and a first transmission line 214 that distributes signals to the first power feeding probes 213 of each of the element antennas.
Ground layers 215 and 216 each having open holes of the same shapes as those of the openings of the first cavity part 201 are disposed on and under the dielectric substrate 211 such that the first transmission line 214 functions as a strip line.
In addition, in order to give a structure similar to that of the cavity part 201 to the inside of the dielectric substrate 211, through-holes 212 of a metal are disposed along the openings of the first cavity part 201 to form cavity sidewalls.
The first transmission line 214 has a start point that is a crossing point with an alternate long and short dash line in the figure, and is connected to an inner conductor of a coaxial line at this point and reaches an antenna lower part piercing through a structure in a −z direction.
The second excitation circuit 220 includes a second power feeding probe 223 configured in a dielectric substrate 221 by a pair of elements to which power is fed in phases opposite to each other for each of element antennas, and a second transmission line 224 that distributes signals to the second power feeding probes 223 of the element antennas.
The second excitation circuit 220 is a structure rotated 90° from the arrangement of the first excitation circuit 210 such that a polarized wave excited by the first power feeding probe 213 and a polarized wave excited by the second power feeding probe 223 are orthogonal to each other.
Ground layers 215 and 225 each having open holes of the same shapes as those of the openings of the first cavity part 201 are disposed on and under the dielectric substrate 221 such that the second transmission line 224 functions as the strip line.
In this case, the ground layer 215 plays a role of a ground of both of the first excitation circuit 210 and the second excitation circuit 220.
In addition, in order to give a structure similar to that of the cavity part 201 to the inside of the dielectric substrate 221, the through-holes 212 of the metal are disposed along the openings of the first cavity part 201 to form the cavity sidewalls.
The second transmission line 224 has a start point that is a crossing point with the alternate long and short dash line in the figure, and is connected to the inner conductor of the coaxial line at this point and reaches the antenna lower part piercing through the structure in the −z direction.
The third cavity part 250 is composed of a metal having open holes.
A D-D′ sectional view of FIG. 18 is shown in FIG. 19.
Here, a lower limit frequency at which the antenna is used is represented as fl, and an upper limit frequency at which the antenna is used is represented as fh.
In this case, it is assumed that a diameter d1 of the first cavity part 201 and a diameter d3 of the third cavity part 250 are equal.
When the antenna is regarded as a square waveguide having the diameter d1, a cutoff frequency fc in a basic mode is given by c/(2×d1), where c is the speed of light.
To enable an electromagnetic wave to propagate through the waveguide at fl, it is necessary to set d1 large such that fl>fc is satisfied.
If a diameter for satisfying fl<fc is used as d1, a cutoff occurs, reflection is deteriorated to thus decrease a gain of the antenna.
On the other hand, when an array antenna is configured using the antenna, to increase a gain of the elements while avoiding radiation in an unnecessary direction at fh, it is necessary to set d0 of an element interval smaller such that d0 is smaller than one wavelength at fh, that is, d0<c/fh is satisfied.
It is evident from the figure that d0>d3 in order to secure a wall thickness between the elements.
In this case, in the configuration of FIG. 19, a width d4 is necessary to dispose the through-holes 212, the first transmission line 214, and the second transmission line 224.
The element interval d0 is a sum of d1 and d4. The element interval exceeds one wavelength at fh.
As a result, a radiation pattern of the array antenna is deteriorated, radiation in an unnecessary direction occurs, and a gain in a desired direction decreases.
As shown in FIG. 20, it is possible to set the diameter d3 of the third cavity part 250 larger than the diameter d1 of the first cavity part 201, and densely dispose the openings. However, even in this case, a relation between d1+d4 and d0 is the same as the above one.
Conversely, when d0<c/fh is satisfied in FIG. 20, the remaining diameter d1 after d4 is secured is cut off, leading to a gain decrease.